Much of the work by R.W. Castenholz and colleagues over the past 6-7 years has involved cyanobacterial communities in Yellowstone National Park or cultures of photosynthetic microorganisms obtained from the Park. The primary focus of these efforts was to elucidate the effects of UV radiation on cyanobacteria—both those protected by the UV-shielding pigment, scytonemin, and others that depend on more active metabolism to repair UV and high light damage. Results have demonstrated that the lower temperature (<35°C) cyanobacteria (i.e. Calothrix spp.) that possess scytonemin in the extracellular sheath show little negative effect of exposure to full solar radiation that includes UV. The degree of UV protection directly correlates with scytonemin content, and adverse field conditions appear to promote higher levels of this protective pigment. Other communities of Synechococcus and Leptolyngbya-like cyanobacteria at higher temperatures (40-47°C) contained no scytonemin or mycosporine-like amino acids. Long-term (2-3 months) exclusion of UV radiation showed little recognizable change in species composition in these communities (as measured by molecular methods), but the UV-minus cyanobacteria were far less competent photosynthetically when exposed to full solar irradiance than those that had continuously been exposed to UV. In another study, the high temperature Synechococcus biofilms (68-74°C) were increasingly inhibited by UV and visible solar irradiance during the course of a clear summer day, particularly at supraoptimal (70°C) and suboptimal temperatures (55°C). However, these populations (also lacking protective compounds) recovered by the following morning, presumably by overnight repair processes.